1. Field of the Invention:
This invention relates to a method for the purification of ethylene oxide obtained by the catalytic gas-phase oxidation of ethylene with a molecular oxygen-containing gas. More particularly, to a process for the recovery of ethylene oxide by the steps of leading to an ethylene oxide absorber, an ethyl oxide-containing reaction product gas resulting from the catalytic gas-phase oxidation of ethylene with a molecular oxygen-containing gas in the presence of a silver catalyst thereby inducing absorption of the gas by an absorbent liquid and recovery of ethylene oxide, then forwarding the absorbent liquid now containing ethylene oxide of an ethylene oxide stripper. The bottom of the ethylene oxide stripper is then heated thereby inducing stripping of ethylene oxide from the top of the ethylene oxide stripper, and leading part of the liquid withdrawn through the bottom of the ethylene oxide stripper to the absorber to be recirculated therein as the absorbent liquid. This invention relates to a method for the recovery of ethylene oxide, which resides in lowering the heating energy of the ethylene oxide stripper, an ethylene oxide refiner and a light ends stripper.
2. Description of Prior Art:
In the step for the recovery of ethylene oxide, the ethylene oxide is obtained by allowing the gas produced by the reaction to be absorbed by an absorbent liquid having water as a main component thereby recovering ethylene oxide in the form of an aqueous solution and causing the aqueous solution to obtain ethylene oxide through stripping. Ethylene oxide is generally recovered as follows. The catalytic gas-phase oxidation of ethylene with a molecular oxygen-containing gas in the presence of a silver catalyst produces a reaction product gas containing ethylene oxide. This gas is introduced to an ethylene oxide absorber allowed therein to come into counterflow contact with an absorbent liquid having water as a main component thereof so as to effect recovery of an aqueous ethylene oxide solution. Then, this aqueous solution is forwarded to an ethylene oxide stripper and, by heating the bottom of the ethylene oxide stripper with steam, ethylene oxide is obtained from the solution. The aqueous solution which now contains substantially no ethylene oxide is withdrawn via the bottom of the stripper to be recirculated as the ethylene oxide absorbent liquid. The diffusate departing from the top of the ethylene oxide stripper and containing ethylene oxide, water, carbon dioxide, inert gases (nitrogen, argon, methane, ethane, etc.), low-boiling impurities such as formaldehyde, and high-boiling impurities such as acetaldehyde and acetic acid is purified by being passed through the step of dehydration, the step of separation of more volatile components, and the step for separation of heavyduty components, to give rise to ethylene oxide. (Refer, for example, to U.S. Pat. No. 3,165,539; 2,771,473; 4,028,070; 3,097,215; 3,217,466; 3,745,092; 3,729,899; 3,766,714; and 3,964,980.)
The method heretofore known to the art will be described specifically below.
With reference to FIG. 1, ethylene is subjected to catalytic gas-phase oxidation with a molecular oxygen-containing gas in the presence of a silver catalyst to produce a reaction product containing ethylene oxide. This gas is passed through a conduit 1 and fed to the lower part of an ethylene oxide absorber 2 in the form of a packed tower or a tray tower. An absorbent liquid is introduced via a conduit 3 into the upper part of the ethylene oxide absorber 2 and brought into counterflow contact in the tower with the reaction product gas to recover not less than 99% by weight of ethylene oxide present in the product gas. Such gases as that portion of ethylene oxide which has not been absorbed, oxygen, carbon dioxide, inert gases (nitrogen, argon, methane, and ethane), aldehydes, and acidic substances departing from the top of the ethylene oxide absorber 2 are forwarded via a conduit 4 and circulated to the carbon dioxide absorption step and/or the oxidation step. In this step of absorption, such lowboiling impurities as formaldehyde and such high-boiling impurities as acetaldehyde and acetic acid which are formed in the step of oxidation of ethylene other than ethylene, oxygen, carbon dioxide, and inert gases (nitrogen, argon, methane, and ethane), ethylene oxide, are immediately substantially completely absorbed. The bottom liquid of the ethylene oxide absorber 2 is passed through a conduit 5 to a heat exchanger 6, there to exchange heat with the bottom liquid of an ethylene oxide stripper and is thus heated to a temperature to 70.degree. to 110.degree. C. The hot bottom liquid of the ethylene oxide absorber 2 is then sent through a conduit 7 to a gas-liquid separation tank 8. The more volatile component of inert gas containing ethylene oxide and water is partly separated via a conduit 9. The absorbent liquid left behind after the more volatile gas has been expelled by flashing is passed through a conduit 10 and introduced to the upper part of an ethylene oxide stripper 11 kept under top pressure of 0.1 to 2 kg/cm.sup.2 G at a top temperature in the range of 85.degree. to 120.degree. C. and heated in a conduit 13 with a heating medium such as stem or a heat medium (produced by The Dow Chemical Company and marketed under trademark designation of "Dowtherm") circulated through a heater 12 annexed to the ethylene oxide stripper 11 or heated directly by feeding to the bottom of the ethylene oxide stripper 11. As the result, not less than 99% by weight of the ethylene oxide contained in the absorbent liquid is stripped. Part of the bottom liquid of the ethylene oxide stripper containing substantially no ethylene oxide and having a temperature of 100.degree. to 150.degree. C. is withdrawn via the bottom of the ethylene oxide stripper 11 and forwarded via conduits 14 and 15 to the heat exchanger 6, thereto exchange heat with the bottom liquid of the ethylene oxide absorber 2. The bottom liquid consequently deprived of heat is passed through a conduit 16 and further cooled by a cooler 17 having cooling water circulated through conduits 18 and 19 therein and passed to absorber 2 via conduits 20 and 3. Then, fresh water is introduced via a conduit 21 for the purpose of adjusting the ethylene glycol concentration in the absorbent liquid. An aqueous potassium hydroxide solution may be added into the absorbent liquid when necessary for the adjustment of the pH of the liquid. For the adjustment of the anti-foam agent concentration in the absorbent liquid, an anti-foam agent may be added into the ethylene oxide absorber 2. To prevent the by-produced ethylene glycol (arising from the hydrolysis of ethylene oxide and water), such low-boiling impurities as formaldehyde, and such high-boiling impurities as acetaldeyde and acetic acid from increasing in the absorbent liquid between the step of oxidation of ethylene with molecular oxygen and the step of stripping of ethylene oxide, the bottom liquid of the ethylene oxide stripper 11 is withdrawn via conduits 14 and 22 through the bottom of the ethylene oxide stripper 11 and forwarded to the step for concentration of the by-produced ethylene glycol.
In the meantime, the vapor containing ethylene oxide and obtained via the top of the ethylene oxide stripper 11 is forwarded via a conduit 23 to a condenser 24 having cooling water circulated through conduits 25 and 26 therein. The condensate consequently produced is returned via a conduit 27 to the top of the ethylene oxide stripper 11 and uncondensed vapor is introduced via a conduit 28 to a dehydrator 29.
The bottom liquid of dehydrator 29 is heated either by being sent through a conduit 31 which is kept heated by a heating medium such as steam or Dowtherm by a reboiler 30 annexed to the dehydrator 29 or directly by the introduction of steam into the lower part of the dehydrator 29. The water, containing substantially no ethylene oxide, is withdrawn via a conduit 32 from the bottom of the dehydrator 29.
From the top of the dehydrator 29, the vapor containing ethylene oxide is forwarded via a conduit 33 to a condenser 34 having cooling water or brine circulated through conduits 35 and 36 therein. The condensate consequently formed is returned via a conduit 37 to the top of the dehydrator 29. The uncondensed vapor in the condenser 34 is introduced via a conduit 39 to an ethylene oxide vent-scrubber (not shown). The remaining part of the condensate in the condenser 34 is introduced via a conduit 38 to a light ends stripper 40.
The supplied liquid is heated by being passed through a conduit 42 heated with a heating medium such as steam or Dowtherm by a heater 41 annexed to the light ends stripper 40. From the top of the light ends stripper 40, the ethylene oxide vapor containing other more volatile component is forwarded via a conduit 43 to a condenser 44 cooled by coolant circulated through conduits 45 and 46. The condensate consequently formed is returned via a conduit 47 to the top of the light ends stripper 40. The uncondensed vapor is introduced via a conduit 48 to an ethylene oxide vent-scrubber (not shown) for the recovery of ethylene oxide.
From the bottom of the light ends stripper 40, the bottom liquid is introduced via a conduit 49 to an ethylene oxide refiner 50.
Steam at a pressure of 0.5 to 3.0 kg/cm.sup.2 G is introduced via a conduit 59 to a heater 58 annexed to the ethylene oxide refiner 50. Rectification is then carried out with the bottom temperature of the ethylene oxide refiner 50 maintained at 35.degree. to 85.degree. C. and the bottom pressure of the tower maintained at 1.2 to 8.2 kg/cm.sup.2 G. The ethylene oxide vapor at a top temperature of 29.degree. to 81.degree. C. and top pressure of 1.0 to 8.0 kg/cm.sup.2 G is withdrawn via the top of the ethylene oxide refiner and forwarded via a conduit 51 to a condenser 52, there to be condensed. Part of the liquefied ethylene oxide is passed through a conduit 56 and introduced as a reflux liquid to the top of the ethylene oxide refiner 50. The remaining part of the liquefied ethylene oxide vapor is withdrawn via a conduit 57 as an ethylene oxide product.
The uncondensed vapor is condensed in the condenser 52 cooled by coolant passing through conduits 53 and 54 of the ethylene oxide refiner 50 and supplied via a conduit 55 to the ethylene oxide vent scrubber (not shown) for recovery of ethylene oxide.
The bottom liquid of the ethylene oxide refiner 50 is withdrawn via a conduit 67 when necessary for the separation of heavy-duty fractions of such high-boiling impurities as acetaldehyde, water, acetic acid etc.
Part of the bottom liquid from conduit 67 is circulated to reboiler 58 where it is heated by a heating medium injected into input 59.
The method of the purification of ethylene oxide described above, however, is not satisfactory in terms of the recovery of the heat of condensation of the vapor liberated through the top of the ethylene oxide stripper and the recovery of the thermal energy possessed by the liquid withdrawn through the bottom of the ethylene oxide stripper. Thus, this method has the disadvantage that a large volume of heat is wastefully discharged from the system. The conventional method has imposed the requirement of causing the bottom liquid of the ethylene oxide stripper which has a temperature of 100.degree. to 150.degree. C. to exchange heat with the bottom liquid of the ethylene oxide absorber thereby effecting recovery of heat and thereafter cooling the bottom liquid and reclaiming the cooled bottom liquid as the absorbent liquid for use in the ethylene oxide absorber. Further, the method for the purification of ethylene oxide has entailed the disadvantage that the heating carried out in the ethylene oxide stripper, the ethylene oxide refiner and the light ends stripper consumes a large volume of heating steam.
The object of the present invention, therefore, is to provide a novel method for the purification of ethylene oxide.
Another object of the present invention is to provide a method for the purification of ethylene oxide, which aims to promote effective utilization of the energy of the bottom liquid in the ethylene oxide stripper and aims to promote effective utilization of the thermal energy of the diffusate from the top of the ethylene oxide stripper.